The present invention relates to pulverized solid fuel (pulverized coal) delivery systems and, more particularly, to a fuel head assembly for use in a pulverized coal delivery system.
FIG. 1 depicts an example of a pulverized solid fuel-fired steam generator 10, which is shown to include a combustion chamber 14 within which the combustion of pulverized solid fuel (e.g., pulverized coal) and air is initiated. Hot gases that are produced from combustion of the pulverized coal and air rise upwardly in the steam generator 10 and give up heat to fluid passing through tubes (not shown) that in conventional fashion line the walls of the steam generator 10. The steam generated in the steam generator 10 may be made to flow to a turbine (not shown), such as used in a turbine/generator set (not shown), or for any other purpose.
The steam generator 10 may include one or more windboxes 20, which may be positioned in the corners or the sides of the steam generator 10. Each windbox 20 is provided with a plurality of air compartments 15 through which air supplied from a suitable source (e.g., a fan) is injected into the combustion chamber 14 of the steam generator 10. Also disposed in each windbox 20 is a plurality of fuel compartments 12, through which pulverized coal is injected into the combustion chamber 14 of the steam generator 10.
The pulverized coal is supplied to the fuel compartments 12 by a pulverized coal supply means 22, which includes a pulverizer 24 in fluid communication with the fuel compartments 12 via a plurality of pulverized solid fuel ducts 26. The pulverizer 24 is operatively connected to an air source (e.g., a fan), whereby the air stream generated by the air source transports the pulverized coal from the pulverizer 24, through the solid fuel ducts 26, through the fuel compartments 12, and into the combustion chamber 14.
FIG. 2 depicts a cross-sectional, elevation view of a conventional pulverized coal nozzle assembly 34 disposed within a fuel compartment 12. While only one fuel compartment 12 is shown, it will be appreciated that each fuel compartment 12 of FIG. 1 may include a nozzle assembly 34. The nozzle assembly 34 includes a nozzle tip 36, which protrudes into the combustion chamber 14, a fuel feed pipe 38, which extends through the fuel compartment 12 and a head assembly 40 by which the nozzle assembly 38 is coupled to the solid fuel duct 26. Typically, the head assembly 40 comprises an elbow that connects the substantially vertical solid fuel duct 26 with the substantially horizontal fuel feed pipe 38.
The nozzle tip 36 may have a double shell configuration, comprising an outer shell 39 and an inner shell 42. The inner shell 42 is coaxially disposed within the outer shell 39 to provide an annular space 44 between the inner and outer shells 42, 39. The inner shell 42 is connected to the fuel feed pipe 38 for feeding a stream of pulverized coal entrained in air through the fuel feed pipe 38 and the inner shell 42 into the combustion chamber 14 (FIG. 1). The annular space 44 feeds a stream of secondary air into the combustion chamber 14, which helps to cool the nozzle tip 36. While the nozzle tip 36 is shown as being separate and pivotable relative to the fuel feed pipe 38, it will be appreciated that the end of the fuel feed pipe 38 may instead be shaped to form a stationary nozzle tip.
Historically, pulverized coal boiler systems have had difficulty achieving uniform distribution of pulverized coal and transport air across the fuel duct 26 and nozzle assembly 34. Maldistribution is associated with the transport in a two phase flow system of a pulverized solid (e.g., coal) and gas (e.g., air). At each turn in the fuel duct 26, separation between the phases occurs. Finally, when the piping transitions from the vertical fuel duct 26 to horizontal nozzle assembly 34, a narrow, concentrated stream of coal, known as a “coal rope”, has been established in certain portions of the cross section of the fuel feed pipe 38.
Each nozzle assembly 34 will have a different coal rope concentration and location depending on the upstream routing of the fuel duct 26 and other factors such as air and coal flow rates. This coal roping promotes localized erosion that accelerates wear and reduces component life. Coal roping also decreases the fuel/air mixing efficiency and, thus, decreases the efficiency of fuel combustion.
The coal ropes cause erosion where they contact the walls. They follow the airflow currents. In FIG. 2, the currents may cause the ropes to erode the walls of the fuel feed pipe 38. The fuel feed pipes 38 are located inside of the fuel compartments 12. These typically pass through the windbox. Therefore, it is very difficult to replace parts within the fuel feed pipe 38.
Other parts, such as the head assembly 40 are exposed and easier to access and maintain.
In the past, improving pulverized coal distribution through the horizontal nozzle assembly 34 was done with a device known as a coal rope breaker, which are typically mechanical devices disposed in the fuel feed pipe 38. For example, U.S. Pat. No. 6,105,516 describes multiple, transversely extending rib segments protruding into the fuel feed pipe portion of the nozzle, U.S. Pat. No. 5,526,758 describes a distribution half-cone mounted within a burner nozzle, and U.S. Pat. No. 5,588,380 describes a conical diffuser with angled support legs disposed along the coal nozzle axis. Another known method for breaking up coal ropes includes placing an orifice within the fuel feed pipe 38.
Experience and computer modeling has indicated that these coal rope breaking devices have had some success in redistributing the air, but little impact on the pulverized coal distribution within the nozzle. Furthermore, these coal rope-breaking devices add unwanted pressure drop to the pulverized coal delivery system. This pressure drop could have the potential of reducing or limiting the pulverizer system delivery capacity.
Thus, there is a need for a device that is easy to service and maintain that improves pulverized coal distribution through the burner nozzle assembly to eliminate or reduce the formation of coal ropes and the problems associated with coal ropes, while reducing the amount of unwanted pressure drop in the pulverized coal delivery system.